As a follow-on to r325378, make the shutdown timer default to 0 as well.

Description

As a follow-on to r325378, make the shutdown timer default to 0 as well.

Otherwise an orderly shutdown will initiate a watchdog that will cause
a 7 minute delayed reboot *by default*, In the freebsd.org cluster's case
this often worked out be a surprise reboot a minute or two after the
machine came back up.